Field of the Invention
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine.
Description of the Related Art
There has been known an NOx selective reduction catalyst (hereinafter, referred to simply as an “NOx catalyst”) which purifies (removes or reduces) NOx contained in an exhaust gas from an internal combustion engine by using ammonia as a reducing agent. At the upstream side of the NOx catalyst, there is arranged an addition valve or the like which serves to add ammonia or a precursor of ammonia into the exhaust gas. As the precursor of ammonia, there can be mentioned urea, for example. Hereinafter, the precursor of ammonia or ammonia is also collectively referred to as “a reducing agent”.
It is desirable that the reducing agent added from the addition valve be dispersed uniformly in the exhaust gas. Here, when the concentration of the reducing agent is not uniform in the NOx catalyst, there is a fear that the rate of NOx reduction may become low at locations where the concentration of the reducing agent is low, whereas the reducing agent may pass through the NOx catalyst at locations where the concentration of the reducing agent is high. For this reason, provision may be made for a disperser which serves to disperse the reducing agent widely in the exhaust gas (for example, refer to a first patent literature). This disperser is provided with a spiral passage which acts to disperse the reducing agent by causing the exhaust gas to swirl.
For example, in cases where an oxidation catalyst, an addition valve, a disperser and an NOx catalyst are arranged sequentially from an upstream side of an exhaust passage, the reducing agent can be quickly dispersed by means of the disperser. In the case of the disperser being not provided, it is necessary to provide a long distance between the addition valve and the NOx catalyst, in order to disperse the reducing agent. For this reason, the distance from the oxidation catalyst to the NOx catalyst also becomes long, so that a long time is taken for raising the temperature of the NOx catalyst, or a larger amount of fuel is required. On the other hand, the distance from the oxidation catalyst to the NOx catalyst can be shortened by the provision of the disperser.
However, in the disperser, the channel cross section thereof for the exhaust gas flowing therethrough generally becomes small, so when the exhaust gas passes through the dispenser, the dispenser provides resistance to the exhaust gas. For this reason, when the flow rate of the exhaust gas is large, a part of the exhaust gas, which can not pass through the disperser, may flow backwards in the exhaust passage. In addition, in the case of the disperser having a spiral passage, the part of the exhaust gas having not passed through the disperser may swirl in the same place.
In this manner, when the reducing agent is contained in the exhaust gas which flows backwards in the exhaust passage or swirls in the same place, there is a fear that in cases where an NOx sensor is arranged at the upstream site of the disperser, the reducing agent may arrive at the NOx sensor. Here, the NOx sensor also detects ammonia as well as NOx. Accordingly, when ammonia arrives at the NOx sensor, the detection value of the NOx sensor increases. For example, in cases where the reducing agent is supplied according to the concentration of NOx, the amount of addition of the reducing agent is made to increase according to the increase in the output value of the NOx sensor. In this case, an amount of reducing agent larger than an amount of NOx existing in the exhaust gas will be added. That is, the reducing agent more than needed will be added, thus causing an increase in the amount of consumption of the reducing agent.
Here, it is known that in cases where the reducing agent may arrive at the NOx sensor under the influence of the pulsation of the exhaust gas, the detection of NOx by the NOx sensor is stopped (for example, refer to a second patent literature). In this second patent literature, it is described that the higher the flow speed of the exhaust gas, the more difficult it becomes for the reducing agent to flow backwards. Then, when the flow speed of the exhaust gas exceeds a specified value, the detection of NOx by the NOx sensor is permitted, even in a period of time in which the reducing agent is added. However, as described above, the exhaust gas may flow backwards under the influence of other factors than the pulsation thereof.